Electrolytic stripping of nickel coatings from ferrous metals



Patented Apr. 17, 1951 ELECTROLYTIC STRIPPING OF NICKEL COATINGS FROMFERROUS METALS Henderson M. Bell, I11, Cleveland Heights, and Ernest W.Schweikher, Chagrin Falls, Ohio No Drawing. Application December 6,1946, Serial No. 714,636

3 Claims. (Cl. 204-146) This invention relates to stripping nickelcoatings; and-it comprises a process wherein a ferrous metal articlehaving a nickel coating is immersed in a stripping bath, comprising anammonium or nitrogen base salt of nitric acid or chromic acid in aconcentration advantageously ranging from about 50 to 300 grams of saidsalt per liter; said bath operating usually at a temperature within therange of about 25 to 55 C. and having a pH within the range of about 3to 6; followed by passing through said bath from said metal article to asuitable cathode a direct current having a current density within therange of about 20 to 400 amperes per square foot, whereby the nickelcoating is stripped without damage to the ferrous base; all as morefully hereinafter set forth and as claimed.

It is frequently necessary in the plating art to strip defective metalcoatings from ferrous metal bases. While several stripping methods areknown, which give more or, less satisfaction in the case of othermetals, the stripping of nickel has always presented great difficulties.Three methods have hitherto been used for thispur pose. One of thesemakes use of a stripping bath containing sulfuric acid at'a densityranging from about 52 to 56 B. The nickel plated parts are made anodesin this stripping bath. A second method makes use of a concentratedneutral solution of sodium nitrate, this bath being operated in the samemanner except for the use of higher current densities. The third methodmakes use of fuming nitric acid without the aid of the elec triccurrent. All of these methods have serious technical and economicdisadvantages and are hazardous in operation.

The sulfuric acid bath has found the widest use up to this time. Howeverthe operation of this bath is much slower than is desired. For example,a nickel deposit that has required from 10 to 40 minutes for itsapplication will need from 30 to 120 minutes for its removal by thisbath. The action of this bath can be accelerated by dilution but, ifthis is done, the attack of the acid upon the base metal is aggravated.This is a step in the wrong direction, for, notwithstanding the use ofaddition agents to inhibit the etching action of the acid, light gaugecold rolled steels are generally etched to a, degree which makes itimpossible to repolish and satisfactorily replate them. Furthermore, aninsoluble nickel sludge is formed which must be removed, resulting inlarge and uneconomic losses of the stripping bath. The disadvantages ofthe sulfuric acid bath are found also in the sodium nitrate bath, whichis even slower in operation.

The fuming nitric acid bath has not been widely used because of theobvious health hazards and J the dangers involved in using so corrosivevan acid.

It is an object of the present invention to provide a stripping bathcontaining an electrolyte which will remove nickel coatings ofcommercial hazard in its use and operation. It isa still further objectto provide astripping composition which' upon, solution in water isready for use without the necessity of any adjustment.

We have discovered that the foregoing and other objects of our inventioncan be attained by the use of an aqueous stripping bath comprising anitrogen. base salt of nitric acid or chromic acid, suflicient acidbeing present in said bath to produce a pH of about 6.0 or below. Theferrous netalarticle whose nickel coating is to be stripped is dippedinto said bath and its coat 7 ing removed by the passage of a directcurrent from the article'to a suitable cathode.

We have found that it is necessary to have present in our stripping baththe nitrate or chromate anion in a concentration not substantially lessthan about 10 grams per liter. Other acids may be present whose anodicdecomposition products are relatively inert or non-activating towardsferrous metals. For example we have found that sulfates and acetates donot unfavorably affect the operation of our bath-s, while chloridescause the etching of the ferrous metal, owing to the activating effectof their anode decomposition product, i. e. chlorine. Chromates andnitrates ofnitrogen bases selected from a class consisting of ammoniumhydroxide, guanidine, diethanolamine, triethanolamine, trimethyl amine,ethylene diamine, urea, benzyl amine, aniline, methylphenyl amine,dimethylphenyl amine, diphenylmethyl amine etc. are suitable.

Our stripping baths can be made by mixing the I amine or ammonia withnitric or chromic acid While the tendency to etch is likewise reduced,the work must nevertheless generally be polished before replatin'g.

in an aqueous medium. For example, a bath can be made by adding nitricor chromic acid and ammonia or an amine to Water, to form thecorresponding ammonium or substituted ammonium salt. It is also possibleand usually more advantageous in cases Where the salts are commerciallyavailable, to add these salts directly to the bath rather than addingtheir reactive components. The resulting baths are operative over a widerange of concentrations and temperatures but for best results theconcentrations, temperatures and pH values must be held withinrelatively close limits.

We have found that if the concentration of the salt is held within therange of from about 10 g./l. (1.3 oz./gal.) to saturation, goodstripping results may be obtained. However, if the concentration isbelow about 50 g./l. (6.7 oz./gal.) the conductivity of the solution isso low that unduly high voltages are required. On the other hand,solutions more concentrated than about 300 g./l.j (40 oz./gal.), whileoperating quite well, do not exhibit sufficiently greater conductivityor stripping speed to justify the additional cost of preparation andmaintenance. In the preferred range, it will be found that a bathcontaining about 200 g./l. (26.6 oz./gal.) of the salt will mostgenerally prove satisfactory.

In the preferred concentration range, the speed at which the deposit isremoved depends upon the temperature and pH of the bath and the anodecurrent density. If the bath temperature is above about 130 F.this limitvarying somewhat with type and quantity of salt and with aciditytheremay be some tendency for anodic steel to etch on prolonged immersion. Itis not desirable to chill the bath, as its resistivity at lowtemperatures is high enough to require uneconomically high voltages. Asa result of these factors temperatures between 80 F. and 130 F. are mostsuitable.

We have discovered that, in order to avoid etching of the ferrous'basemetal, it is essential that the stripping bath be maintained at leastslightly on the acid'side of 'the neutral point. Thus, we have foundthat a neutral stripping bath containing ammonium nitrate'causes someetching of steel when'the latter is merely immersed in the bath for anylength of time. But surprisingly we have discovered that this etching iseliminated by slightly acidifying the bath and passing an electriccurrent therethrough from the steel to a suitable cathode. Itwould', ofcourse, have been expected that either or both of these changes wouldhave' accelerated the etching. The fact is, however, that the'com binedeffect of the electric current and a slightly acid bath sharplydiminishes the etching eife'ct At pH values only slightly on the acidside of the neutral point, that'is, at a pH of 6.0 or above, we havefound that the stripping rate isfqu'ite' within a pH range of about 3.0to 6. This avoids both difficulties.

In operation the pH of the stripping bath tends to increase; henceadjustment of the pH from time to time is necessary. The pH can becontrolled within the desired limits by occasional addition of, an acidor'an acid salt. It. is usually suitable materialsjare'l synth t ejresins or the: vinyl or vinylidene groups,"poly s'tyrene',polyethyladvisable to add in this manner the acid whose salt is presentin the bath in order to be certain that the passivating character of thebath shall be preserved. But we have found that it is also feasible toadjust the pH, especially the initial pH, by the addition of sulfuricacid or sodium acid sulfate. This reduces the operating cost but, ifsuch an acid is used during operation of the bath, tests should be madefrom time to time to be certain that the concentration of thepassivating anion in the bath does not fall substantially below about 10g./l. since the passivating acid is used up in the process. When thepassivating acid is added to control the pH, however, usually it is notnecessary to test the concentration of this acid.

A very convenient way of starting the operation of one of our strippingbaths is to use a mixture of dry chemicals which when dissolved in waterin a predetermined quantity will produce a stripping bath of the correctpH and concentration. The acid constituent employed in such compositionsmust, of course, be dry. Chromic acid and sodium acid sulfate are themost convenient to Thus a mixture of ammonium nitrate with eitherchromic acid or sodium acid sulfate can be' employed and a mixtureiof, aIl'll-i monium or a substituted ammonium chromate with potassiumbisulfateor chromic acid is equal; ly satisfactory.

We have found no upper limit to the current, density that may be used.Densities as high as 528 a./sq. ft, (56.7 a./sq. dm.) work quite as,well as densities as low as 20 a./sq. ft. (2 .1 a./sq. dm.).

Very high current densities will of course demandhigh voltages and thespray from the cathodesis likely to be excessive, with loss ofstripping. solution. Onthe'other hand, too low an anode current densityresults in an unduly extended stripping time. Moreover, very lowcurrent'densities,

or no current density atall may result in etching of the work; Suitablecurrent densities may be obtained from customary direct current sources,

ing conditions for our stripping "bathfinay' be tabulated as follows:

Concentration of Salt, 50 g ./l. (6.7.o z,/gal.). to.30.0.

g./l. 40 oz. ga1.) pI-I, 3.0 .to 6.0 Temperature, 25? C. (77?. F.)- to5530, (131? Current densities, 20 a./sq. ft. to, 00 a./s q. ft; (or. 2.0a./sq. dm. to 40 a,/sq.'d1n,)

A tank made of corrosion-resistant material .or.

lined with such, materiallis requiredfmmongj ene or other plastics, and,glass. It'is necessary merely to dissolve. the salts in, water in such aI tank, connectthe worr' b the positive pole of a generator orothersourceof cur rent, and immerse it' the s olution, the nega} tive We u ent u e .bQiflQQQHIEQWd;

to an inert qeth eaj ft el' qn iq l or. x: ample, mmersed in he ,$Q l.QI!-

Our invention canbeexplained more fully, by,

reference to the following-specific examples which CBSS.

Example 1 A solution is made up containing:

nrnnos g./1.

NaHSO4 enough tolower the pH to 4.0

In this solution nickel having a thickness of 0.0016 inch was completelystripped in 9 minutes at 3.3 -a./sq. in. current density from polishedsteel, leaving it bright, and in condition tobe replated withoutrebuifing.

Example 2 I Triethanolamine I. 90 g./l. HNOs enough to lower the pH to4.0

In'this solution, nickel having a thickness of 0.0016 inch wascompletely stripped in minutes at l".3"'a./sq. "in. current density]from polished steel,"1eaving' it bright, and in condition to be replatedwithout rebufling.

, Example'3 ole/1.

NHQOH enough to raise the pI-Iito 4.0

HNO3 adding during, stripping to maintain pH Example ,4

'lojo g./1. rmos enough to pH to 4.0

In this solution, nickel having a thickness of 0.00125 inch wascompletely stripped in 40 minutes at a current density of 0.75 a./sq.in. from polished steel, leaving it bright, and in-condition to bereplated without repolishing..

Example 5 NH4NO3 200 g./1. NaHSO4 0.6 g./1.

lower the Example 6 Urea 150 g./l. CrOs 7 g./1. HNO3 enough to lower thepH to 5.0

In this solution, nickel having a thickness of 0.00185 inch wascompletely stripped in 14 minutes at a current density of 1.3 a./sq. in.from polished steel, leaving it bright and in condition to be replacedwithout repolishing.

Example 7 Guanidine carbonate 110 g./l. HNO3 ml./l.

This is equivalent to a 150 g./l. solution of guanidine nitrate. Becauseof the lesser solubility of guanidine nitrate at lower temperatures, thebath was maintained at -130 F. The pH was kept between 3.5 and 6.0 byoccasional additions of HNOs. In this solution, nickel having athickness .of- 0.00175;inch was completely stripped in- 13. minutes at acurrent density of 1.8 a./sq. in-

from polished steel, leaving it bright and in condition to be replatedwithout repolishing.

Example 8 Ethylene diamine (100%) 18.75 g./l.

CIO3 31.25 {3/1.

HNOs enough to lower the pH to 5.5

In this solution, nickel having a thickness of 0.0017 inch wascompletely stripped in 27 minutes at a current density of 0.75 a./sq.in. from pol- In this solution, nickel having a thickness of 0.0005 inchwas completely stripped in 10 minutes at a current density of 1.6 a./sq.in. from polished steel, leaving itcoated with a soft thick black filmMost of the'film could be swabbed off, and after a 10 second dip indiluted HCl, the piece Was found to be bright and in condition to bereplated without repolishing.

It is believed evident from the above that dry mixes can be'made fromthe solid components of the baths illustrated in the above exampleswhich, when dissolved in water, will produce stripping baths within ourinvention. Thus chromic acid and sodium bisulfate, for example, can beused I as the solid acid constituents while nitrogen base nitrates,chromates, dichromates, carbonates etc. can be used as thenitrogen-containing constituent. V

A specific example of a dry mix which, upon solution in water, willproduce one of our stripping baths, is a mixture of ammonium nitrate andsodium bisulfate in the proportions of 99.7 parts of the nitrate to 0.3part of the bisulfate. This mixture, when dissolved in Water to producea solution containing approximately 200 grams per liter, produces a pHin the neighborhood of 3. Another dry mix can be made by mixing together99.98 parts of ammonium nitrate with 0.02 part of chromic acid. Thismix, when dissolved in water to produce a solution containingapproximately grams per' liter, produces a pH of about 4.2

It is of interest to note that the action which takes place during thestripping operation of our invention differs to some extent-at least inappearance-depending upon the nitrogen compound employed. For example,in the stripping of nickel in accordance with Example 1, a film of heavyliquid appears to fall away from the surface of the anode at the startof the process. The surface of the nickel becomes black. No gas isevolved from the anode. After a comparatively short time, breaks occurin the nickel surface and oxygen is evolved Where the underlying steelis exposed. The stripped areas expand and the edges of the nickeladjacent these areas start to curl. After a further short time, the'bits of blackened nickel are dislodged from the anode by the evolvinggas. When all the nickel islands are either eaten up or floated away,oxygen is liberated all over the anode area. Electrolysis may becontinued for a considerable time after all the nickel is gone, withoutany important change in the appearance of the anode. In the case ofExample 6, the nickel does not blacken,

angers but holes are eaten into it, stopping at-thesteel;

and these holes expand, eating away at the edges' ofthe remaining.nickel.

It will. be: appreciated that the chief merits of our process are (1)that it will rapidly dissolve nickel and (2) that iron can be subjectedto it for extendedperiods of time without damage. We know of no otherstripping process'that possesses both advantages.

While we have described what we consider to be the most advantageousembodiments ofour process, it is evident, of course, thatvariousspecific details which have been described can be varied withoutdeparting fromthe purview of our invention. The procedure to be used inmaking up our stripping baths will depend, of course, uponthe relativecost and. availability of the inredients. ample, and the cost of theamine plus the: cost ofv the oxidizingacid is lowerxthan the costoftheamine salt, it would be preferable to make the stripping baths byadding, the amine audthecxidizing acid separately to the water thusforming the amine. salt in situ. This is a very satisfactory way toproceed since, if the amine is added first the oxidizing. acid can beadded until the pH of the bath is within the desired range. On the otherhand, if the amine salt is added to the water, itis then usuallynecessary to'adjust the pH by the addition of'an acid or acid salt. Anyconventional type of electroplating or electrostripping apparatus can beused in our process and conventional stripping procedures can be used;Other modifications of'our. inventionwhich fall within the scope of thefollowing claims will be immediately evident to those skilled in thisart.

What we claim is:

1. In the. stripping of nickel from. ferrous ing a concentrationrangingfrom' about 50' to 300' grams per liter. and a pH'of about 3.0to.6.0.

2'. In. the stripping of nickel from ferrous:

metalS the process which comprises inserting a;

If an. amine. salt is to be used, for ex-- 8 ferrous metal articlecoated with nickel in a stripping bath andpassing a direct current fromsaid article, serving as anode, to a cathode; said stripping bathconsisting substantially of' arr aqueous solution of asalt whose anionis that of an oxidizing-passivating acid selected from a classconsisting of nitric" and chromic acids and whose: cation is that of anitrogen base selected froma class consisting of ammonium hydroxide,

"guanidine, diethanolamine, triethanolamine, trimethyl amine, ethylenediamine, urea, benzyl amine, aniline, methylphenyl amine, dimethylphenylamine and diphenyhnethyl amine, said bathihaving. a concentration ofsaid salt ranging fromabout to 300 grams per liter and a pH within therange of from about'3.0 to'6.0'.

3. In the strip ing of nickel from ferrous metals; the process whichcomprises inserting a ferrous metal article coated with nickel in astripping bath and assing an electric current from said article,serving; as anode, to a cathode; said stripping bath consistingsubstantially of an aqueous solution. of ammonium chromate havingaconcentration ranging from about 50 to 300 grams per liter and a pH offrom about 3.0 to 6.0'.

HENDERSON M. BELL ERNEST SCHWEIKHER.

B EFERENCES CITED The following references are of record in ..the"- fileof. this patent:

UNITED STATES Number Name Date 908,937 Bayliss et a1 Jan. 5, 1909'1,314,842: Weeks.. Sept. 2, 1919 2,241,585 Day May 13, 1941 OTHERREFERENCES The Monthly Review of The-American Elec-- troplaters Society,July 1945,.pages 673, 679.

Metal Cleaning and. Finishing, Nov. 1933,-

2. IN THE STRIPPING OF NICKEL FROM FERROUS METALS, THE PROCESS WHICHCOMPRISES INSERTING A FERROUS METAL ARTICLE COATED WITH NICKEL IN ASTRIPPING BATH AND PASSING A DIRECT CURRENT FROM SAID ARTICLE, SERVINGAS ANODE, TO A CATHODE; SAID STRIPPING BATH CONSISTING SUBSTANTIALLY OFAN AQUEOUS SOLUTION OF A SALT WHOSE ANION IS THAT OF ANOXIDIZING-PASSIVATING ACID SELECTED FROM A CLASS CONSISTING OF NITRICAND CHROMIC ACIDS AND WHOSE CATION IS THE OF A NITROGEN BASE SELECTEDFROM A CLASS CONSISTING OF AMMONIUM HYDROXIDE, GUANIDINE,DIETHANOLAMINE, TRIETHANOLAMINE, TRIMETHYL AMINE, EHTYLENE DIAMINE,UREA, BENZYL AMINE, ANILINE, METHYPHENYL AMINE, DIMETHYLPHENYL AMINE ANDDIPHENYLMETNYL AMINE, SAID BATH HAVING A CONCENTRATION OF SAID SALTRANGING FROM ABOUT 50 TO 300 GRAMS PER LITER AND A PH WITHIN THE RANGEOF FROM ABOUT 3.0 TO 6.0.